1. Field of Invention
The invention relates to a contact structure for compound semiconductor device, and particularly to a group III-V compound semiconductor device generally used as a light emitting diode (LED), a laser diode (LD), or a photodiode (PD), and more specifically to a contact structure for group III-nitride, group III-phosphide, and group III-arsenide based LED, LD, and PD. The compound semiconductors satisfy the following general formula: AlxGayIn1xe2x88x92xxe2x88x92yN, wherein 0 less than =x less than =1, 0 less than =y less than =1, and 0 less than =x+y less than =1 inclusive; AlxGayIn1xe2x88x92xxe2x88x92yAszP1xe2x88x92z, wherein 0 less than =x less than =1, 0 less than =y less than =1, 0 less than =x+y less than =1, 0 less than =z less than =1 inclusive.
2. Description of the Related Art
Group III-V compound semiconductors have been used to make many electro-optic and opto-electronic devices including LED, LD, and PD. For these devices, in addition to the desire to have good crystal quality, it is recognized in the invention that there is a desire to have contact electrodes with both low contact resistance to the semiconductor and a conductive layer having low sheet resistance substantially over the area of the semiconductor. Lower contact resistances allow reduced energy dissipation at the contact region. Lower sheet resistances ensure an even spreading of current in the lateral direction of the semiconductor surface such that substantially the entire active region of the device may be utilized. When III-V compound semiconductors are used for such light-sensitive devices as LED, LD, and PD, the spread out conductive layer of the contact should also allow light emitted in the active region to propagate out of the device by substantially passing through the layer with minimum loss. It is desired then to have a contact structure for allowing substantially the entire active region of the semiconductor to be utilized for light emission, while still allowing the generated light to escape from being blocked or absorbed by the contact as it tries to exit the device.
There exist techniques directed toward providing devices having characteristics in accord with the desired features briefly described above, i.e., low contact and sheet resistance, and high transmission efficiency of generated radiation, and solving recognized contact problems for group III-V semiconductor LED. In one reference, Shibata Naoki, et. al., at U.S. Pat. No. 6,008,539, disclose an electrode structure for a GaN based compound semiconductor LED. FIG. 1 of the present application schematically illustrates one of the structures described by Naoki et al. Referring then to FIG. 1, an LED structure is shown including a sapphire substrate 1, an AlN buffer layer 2, a Si-doped n+-type GaN layer 3, an undoped n-type AlGaN layer 4, a Mg-doped GaN active layer 5, a Mg-doped p-type AlGaN layer 6, a highly Mg-doped p+-type GaN layer 7, and a double layer (Ni/Au) electrode 8A which contacts the top p+-layer 7, and an electrode 8B which contacts the n+-type layer 3. Naoki et al. explain that the layer 8A contacting the p+-type layer 7 acts as a contact electrode with low contact resistance and sheet resistance. However, the device shown at FIG. 1 and described by Naoki et al in the ""539 patent does not have good optical transparency, and therefore, exhibits poor LED light output efficiency. Even if the Ni/Au stack layer thickness is as thin as 40 Angstroms/40 Angstroms, the contact electrode still absorbs approximately 40% of the light generated, as estimated by R. W. Chuang et. al. in a similar LED structure published in MRS Internet Journal of Nitride Semiconductor Research, 4S1, G6.42 (1999), incorporated by reference below.
In another related art reference, Biing-Jye Lee, et. al., at U.S. Pat. No. 6,057,562, disclose a layer structure for a group III-V compound semiconductor AlGaInP LED. As schematically illustrated at FIG. 2 of the present application, the LED structure proposed by Lee et al. includes a back contact 50 on a substrate 52 with a Distributed Bragg reflector layer 80, an active layer 54 of stacked AlGaInP, a 10 micrometer thick III-V compound semiconductor window layer 56 and an indium-tin-oxide layer 60 sandwiching a p-type semiconductor contact layer 58, and a top electrode 62. Current spreading is achieved by the combination of window layer 56 and the conductive transparent oxide layer 60, in conjunction with a highly doped p-type III-V compound semiconductor layer 58 to attempt to achieve a substantially ohmic contact between layers 56 and layer 60. Improved light output efficiency was observed by Lee et al. with this LED structure over that realized by Naoki et al. However, many additional process steps, such as metal organic vapor phase epitaxial processes, are involved for incorporating the Distributed Bragg Reflector layer 80, as well as for forming the thick window layer 56 and contact layer 58, which undesirably increase manufacturing complexity and cost
It is therefore a first object of the invention to provide a contact structure with both low contact resistance and low sheet resistance such as to utilize a substantial portion of an active region for compound semiconductor, preferably of group III-V type, LED, LD and PD devices having p-type and/or n-type conduction.
It is a second object to provide a contact structure with high optical transparency such as to efficiently transmit light generated in the active region of the device.
A third object of the present invention is to provide an efficient fabrication method for a high light output compound semiconductor LED and LD in accordance with the first and second objects.
Accordingly, a compound semiconductor LED or LD or PD is provided with a contact structure including a thin metal layer and a transparent conducting oxide layer. The thin metal layer provides a low resistance direct contact to the semiconductor and is preferably formed of at least one of Indium (In), Tin (Sn), nickel (Ni), Chromium (Cr), and Zinc (Zn), or an alloy or multilayer structure of two or more of these elements. The transparent conducting oxide layer is preferably in direct contact with the first thin metal layer and exhibits low sheet resistance for current spreading resulting in enhanced LED light output and is preferably formed of at least one of Indium Tin Oxide (ITO), Indium oxide (InO2), and Tin oxide (SnO2), or an alloy of two or more of these transparent conducting oxides. A conductive pad, preferably a metal, in contact with a portion of the transparent conducting oxide layer is preferably formed of at least one of Al, Au, Cr, Mo, Ni, Pt, Pd, Rh, Ta, Ti, and preferably provides a contact pad for wire-bonding or otherwise connecting the semiconductor device to external circuitry. Preferably the compound semiconductor device is either a group III-V or a group II-VI semiconductor device.
Preferably, the thin metal layer in contact with the semiconductor on one side and with the transparent conducting oxide layer on the other side is compatible with both the semiconductor and the transparent conducting oxide layer. Preferably, interdiffusion of the elements of the semiconductor-metal-transparent conducting oxide multilayer structure occurs for enhancement of the performance of the semiconductor device.
Also preferably, the semiconductor-metal-transparent conducting oxide multilayer structure is subjected to a thermal anneal process at an elevated temperature which causes alloying between the multilayer contact structure and semiconductor layer for further reduction of the contact resistance and sheet resistance, and wherein the optical transparency and adhesion of the contact are further enhanced.